The removal of heavy metals from water is an important aspect of water treatment. There are many technologies for accomplishing this; however, one of the most cost effective means is chemical precipitation.
“Chemical precipitation”, as used herein and generally in the art, refers to reacting dissolved metals with an additive chemical of some sort so that the metals to be removed are rendered insoluble, so that they can then be separated from the water. Raising the pH to a neutral or an alkaline level will precipitate most heavy metals as metal hydroxides. However, hydroxide precipitation is usually not effective enough to meet strict new discharge limits. Metal hydroxides are not insoluble enough to meet these limits and metal ions that are chelated usually will not precipitate at all. Therefore, more advanced treatments such as reaction with organic or inorganic sulfides must be used. These chemistries will produce metal sulfides that have lower solubility than hydroxides and will break chelate bonds to allow the metals to precipitate.
The Department of Army Engineering and Design Manual No.1110-1-4012 on page 2—2 (Precipitation/Coagulation/Flocculation), shows the difference between the solubility of metal hydroxides and metal sulfides. Under ideal conditions, the optimum metal hydroxide solubility ranges from 102 to 10−2 mg/L. Under ideal conditions, the optimum metal sulfide solubility ranges from 10−2 to 10−12 mg/L.
If all the metals (chelated and non-chelated) are precipitated with sulfide chemicals in a one-step precipitation the removal is complete but the cost of treatment is high and often prohibitively high for waste streams containing high concentrations of heavy metals. If most of the metals are first removed as metal hydroxide in a first-step precipitation, and the remaining metals are polished out in a second-step precipitation (sulfide) the removal of metals is improved and the cost of treatment is much lower. In order to do this effectively, this present invention shows it is beneficial to use selected “field separation” methods that has not been used or contemplated before in combination with this two-step precipitation process (hydroxide and sulfide).
The concept of removing heavy metals using sulfides and ferrous was described by Anderson in U.S. Pat. No. 3,740,331. However, Anderson fails to suggest refinements and additions provided by the present invention that make this basic technique improved in today's processing environment. Specifically, Anderson does not suggest that removing metals can be made more efficient if the heavy metals are removed in a two-step precipitation process. The teachings of the Anderson patent are simply that using ferrous with sulfide will result in better metal removal. No suggestion is made to use “field separation” methods that are effective in removing fine and fragile metal precipitates.
The fundamental disadvantage of doing a sulfide precipitation according to Anderson is that it produces very fine colloidal particles that are hard to remove. The present inventor attempted to remove these particles with a sand filter or with a one micron sized back washable filter and was unsuccessful.
The teachings of Fender in U.S. Pat. No. 4,422,943 describe the benefits of using iron pyrite as a source of sulfide to precipitate heavy metals as metal sulfides. He also describes the benefits of using a two-step precipitation process. In his claim #2, he describes the step of separating said precipitated sulfides by filtration (specifically sand filtration), but does not contemplate using the “field separation” methods described in this present invention. However, to accomplish filtration, he uses a polymer to increase the particle size so the sand filter can remove the metal sulfides. It is known in the art that using an organic polymer to increase the size of the metal sulfide precipitates will cause fouling problems with a sand filter. Therefore, the present inventor concludes that his approach is improved because the “field separation” methods covered by this present invention are not subject to fouling like filters. Also, sand filters have a limitation on the size of particles they can remove. A well designed multi-media sand filter can remove particles only down to about 20 micron in size. Metal sulfide precipitation will produce colloidal sized particles of less than one micron in size and these particles will pass through a sand filter. With the exception of microfiltration which can remove sub-micron sized particles, the present inventor has found no filtration equipment capable of consistently and economically removing fine metal sulfide particles. The present inventor has experimented with a back washable filter manufactured by Asahi. It had a plastic-mesh filtering element with a one micron opening size. This was significantly smaller than the metal sulfide precipitates which were at least 30 micron because they were visible to the naked eye. However, the present inventor learned that even at low operating pressures (about 10 psi), the pressure was enough to deform the shape of the metal sulfide precipitates and force these >30 micron sized particles through one micron sized openings.
The only commonality between this present invention and the Fender patent is they both recognize the economic importance of using a two-stage precipitation process, which is known art. In summary, this present invention deals with other forms of soluble and insoluble sulfide treatment rather than iron pyrite and “field separation” equipment rather than filters, which is an improvement to the Fender patent. The Fender patent only deals with iron pyrite as a source of sulfide to precipitate heavy metals. This patent deals with other sulfides that are known to produce small metal sulfide particles that are difficult to filter.
There is a difference between filtration equipment and “field separation” equipment as discussed in the Chemical Engineering document Dated February 1997, Volume 104, Issue 2, Page 66. Filtration equipment includes: straining, cake filtration, deep bed filtration, and membrane filtration and always involves a barrier that prevents the passage of specific sized particles. “Field separation” includes: gravitational settling, centrifugal settling, hydrocyclone separation, dissolved air flotation, expanded plastics flotation, and magnetic separation. The difference is filtration involves a physical barrier to trap particles while “field separation” involves force-fields like molecular, gravitational, centrifugal, and magnetic to separate particles from water.
U.S. Pat. No. 6,099,738 to Wechsler deals with a method and system for removing solutes from a fluid using magnetically conditioned coagulation. This method includes the steps of magnetically conditioning the fluid by applying a magnetic field to enhance the precipitation of solute particles for coagulation; adding a coagulant to the fluid before, during, and after application of the conditioning magnetic field to coagulate the increased available solute particles to form colloids; and collecting the colloids for removal from the fluid. Wechsler neither contemplates combining magnetic seeding and polymer addition with a two-step metal precipitation process as a means for efficiently removing heavy metals from wastewater, nor combining magnetic separation principles with gravity settling in one treatment vessel as described herein.
In this present invention, any magnetic separation method can be used; however, the preferred embodiment of this present invention is novel because the magnetic separator used to capture the magnetic particles are mounted in the treatment tank rather than as a separate collection device. This approach has three advantages: (1) one less piece of equipment is needed, (2) the system can be cleaned without interrupting the water flow, and (3) permanent magnets can be used rather than electromagnets.
Magnetic seeding is used according to this present invention to remove precipitated pollutants and other non-magnetic particles from water. Magnetic seeding is known per se for such purposes. Specifically, the Department of Energy published studies (C. Tsouris, et. al., Electrocoagulation for magnetic seeding of colloidal particles, Physiochem Eng. Aspects accepted paper December 1999; C. Tsouris, et. al., Flocculation of paramagnetic particles in a magnetic field, Journal of Colloid and Interface Science, 171, 319-330; T-Y Ying et. al., High-gradient magnetically seeded filtration, Chemical Engineering Science 55 (2000) 1101-1113) addressing the effectiveness of magnetic seeding to remove colloidal sized particles. The DOE investigators studied magnetically seeded solid/liquid separation by combining magnetic seeding under turbulent-shear flow and high gradient magnetic filtration. They concluded that magnetic seeding was effective in removing fine particles. They used seed particle concentration, solution pH, and ionic strength parameters that determine the zeta-potential of particles to significantly affect the particle removal performance. They did not use organic polymers to bind the magnetic seed materials to the low-magnetic particles to enhance removal, and did not apply magnetic seeding and filtration principles to the second step of a two-step metal precipitation process using sulfide precipitants.
In researching the present patent work, the present inventor found that a strong enough bond between the magnetic seed material and the non-magnetic metal sulfide precipitates to enable reliable separation could not be achieved unless a flocculating polymer was also used. The polymer binds the magnetic seed material together with the fine metal sulfide particles so they can be removed by a low field strength magnetic separator or by gravity settling.
Another novel approach of this present invention is the removal of fine precipitates in the second step of this two-step precipitation process by the use of expanded plastics to enhance flotation. The present inventor successfully attached fine metal precipitates to expanded polystyrene (EPS) with a flocculating polymer. Then the EPS floats carrying the metal precipitates out of the water stream.
The concept of enhanced flotation using highly buoyant ESP is similar to the principle used in DAF (Dissolved Air Flotation) equipment. DAF uses micro-bubbles to float fine particles out of water while the present invention uses an expanded plastic like ESP. The advantage of the present invention is no energy is needed to compress the air and wastewater to form the micro-bubbles.
To date, two-step precipitations (hydroxide and sulfide) have been rarely used because they require additional equipment and space. This level of treatment was not necessary because existing regulatory limits could be achieved with a one-step hydroxide precipitation. However, with tighter regulations, a two-step precipitation process is now justified but the traditional clarification approach is often infeasible because of the high residence times required which causes cost and space limitations.
The present inventor has done a patent and literature search and can find no reference to any of the methods described in this present invention. This present invention describes better ways to do a two-step precipitation that is less costly and smaller in size than a traditional clarifier yet able to handle the metal precipitates in a gently way that prevents their breakup.